The subject matter disclosed herein relates to motor-compressors, in particular to centrifugal motor-compressors and more specifically to multi-stage motor-compressors, particularly multi-stage centrifugal motor-compressors.
Motor-compressors are widely used in several industrial applications to boost the pressure of a gas. Motor-compressors usually comprise a casing wherein a rotor shaft is rotatably supported. One or more impellers are mounted on the rotatable shaft for rotation therewith. A gas enters the compressor at an inlet manifold and is delivered through an outlet manifold at a higher pressure. The work required for boosting the pressure of the gas is provided by a prime mover, for example an electric motor, the motor shaft whereof is mechanically connected to the rotor shaft of the compressor. In known arrangements, the electric motor can be arranged outside the casing of the compressor or integrated in one and the same casing which also houses the compressor stages. In multi-stage compressors the motor drives into rotation all the impellers of the motor-compressor.
FIG. 1 illustrates a compressor 100 of the current art, driven by an electric motor arranged outside the casing of the compressor and not shown. The compressor comprises a compressor casing 101 with an inlet manifold 103 and an outlet manifold 105. A rotor shaft 107 is rotatingly supported in the casing 101 between bearings 109 and 111.
The compressor 100 of FIG. 1 is a two-stage centrifugal compressor comprising a first impeller 113 and a second impeller 115 mounted on shaft 107 and rotating therewith in the casing 101. A first diffuser 117 associated to the first impeller 113 and a second diffuser 119 associated with the second impeller 115 are provided in a stationary position in the casing 101 of compressor 100. A bladed return channel 121 returns the gas delivered by the first impeller 113 through diffuser 117 towards the inlet of the second impeller 115. Gas delivered by the second impeller 115 is collected by a volute 123 and finally discharged through the outlet manifold 105.
The return channel 121 as well as the diffuser 117 and the duct 119 are formed in a stationary diaphragm 125, arranged in the casing 101.
The rotor shaft 107 is coupled, for example through a gear box 108, to an electric motor, not shown. Sealing arrangements must be provided on shaft 107 to prevent gas processed by the compressor from escaping the casing 101.
A balancing drum 116 can be mounted on the shaft 107 or formed integrally therewith, in order to at least partly compensate the axial thrust generated by the gas flow being processed on shaft 107.
In order to remove the need for sealing arrangements on the rotor shaft of the compressor and to reduce the footprint of the motor-compressor arrangement, embedded electric motors have been suggested in combination with the compressor stages of the centrifugal compressor.
U.S. Pat. No. 5,547,350 discloses a modular shaftless motor-compressor, wherein each single impeller is driven into rotation by an embedded electric motor, having a motor stator supported on a fixed portion of the casing and surrounding a first, gas inlet chamber coaxial with the impeller. A motor rotor is arranged around the motor stator, rotates integrally with the impeller and surrounds the gas inlet chamber. The motor rotor is also provided with bearings rotatingly supporting the motor rotor and the impeller in the stationary casing. Each module of the shaftless motor-compressor according to this known prior art has an axial extension which exceeds the axial extension of the impeller, since the embedded motor is arranged in front of the impeller and increases the overall axial dimension of the stage. The diffuser is stationarily arranged in the compressor casing and extends from the outlet of the impeller radially outwardly and towards a respective return channel.